1. Field of the Invention
The present invention relates to a frame transmission scheduling method of a wireless communication considering frame transmission delay time information.
2. Description of the Related Art
Wireless local area networks (LAN) are widely used in a variety of wireless user environments such as home networks, enterprise
e wireless networks and hot spots. A legacy commercial wireless LAN is an extension of the Ethernet, which provides only best effort services based on IEEE 802.11b standardized in 1999. However, wireless LAN users desire faultless transfer of multimedia streams without losing transmission data. Particularly, even in a wireless LAN environment, superior quality of service (QoS) is indispensable to new applications such as video or multimedia streaming.
Continuous desires of users for bandwidth expansion cause increased congestion and decreased relative transmission speed of the entire wireless network. Accordingly, a network manager comes to need a new mechanism to guarantee services of applications that require strict QoS even in a network with high congestion. Such requirements result in the development of a further enhanced media access control (MAC) protocol than in the conventional LAN.
802.11 MAC defines a mandatory function of distributed coordination function (DCF) and an optional function of point coordination function (PCF). That is, a transmission medium can operate both in contention mode of DCF and in contention free mode of PCF. DCF is an asynchronous transmission method, which provides a basic medium access method of 802.11 MAC and has been implemented in all kinds of commercial wireless LAN products. In terms of wireless medium access, DCF does not consider priorities between stations (terminals, hereinafter referred to as ‘STA’) at all. Such a characteristic of DCF does not reflect transmission of various types of data traffic, and thus cannot support QoS requested by users in the end.
A synchronous transmission method is a medium access method based on a polling mechanism, which is implemented through PCF. In PCF, a function of a point coordinator (PC) is placed at a central access point (hereinafter, referred to as ‘AP’), and the AP directly controls all services provided to STAs in a centralized polling scheme. That is, the AP periodically polls connected STAs to give an opportunity to transmit frames to the STAs.
Legacy 802.11 MAC has many problems in supporting wireless LAN QoS. The DCF, i.e. a mandatory function of 802.11 MAC, does not provide any function of supporting QoS. Accordingly, when a DCF method is used, all data traffics are serviced in order of arriving at a transmission queue and processed in best effort mode.
Contrary to the DCF, PCF of 802.11 MAC has been developed to support real-time traffic services but currently supports QoS. However, the PCF has the following problems.
That is, in PCF, the PC placed at the AP defines a scheduling algorithm for the purpose of polling simply based on a round-robin method. However, there are practically various types of traffics that require differentiated QoS, and thus, the round-robin algorithm that cannot assign a priority to traffic is insufficient for supporting QoS.
Further, there is a problem in that if the size of a super frame is small, alternations between a contention period and a contention free period can lead to a considerable overhead.
Further, in legacy MAC, transmission of beacon frames or a starting point of a super frame can be changed. The PC prepares a beacon frame that should be transmitted after a target beacon transmission time (TBTT), and then transmits the beacon frame if the medium is idle as long as a point inter-frame space (PIFS). However, even though STAs cannot complete transmission of frames before a subsequent TBTT, they even can start to transmit frames. Therefore, there is another problem in that the transmission of beacon frame can be delayed.
The delay of beacon frame which should be transmitted immediately after the TBTT delays the transmission of time-constraint frames which should be transmitted within the contention free period. Such a problem causes time delay that is difficult to estimate in a contention free period and thus has a serious influence on QoS.